This invention pertains to a particle classifier apparatus and method for separating solid particles carried in a gas stream according to their particle size. It pertains particularly to a particle classifier apparatus and method used for separating and removing fine coal particles carried in an air stream as a fuel feed stream for firing a boiler, and returning larger coal particles to a crusher for further size reduction.
Particle classifiers of various types such as coal classifiers have been known and used in the power industry for many years, and are located between a coal crushing or grinding mill and the pulverized coal feed as fuel to steam boilers. Such classifiers control the particle size of the coal feed to the boiler combustion zone to a desired size range. The classifiers typically utilize centrifugal and gravity forces on the particles to achieve a selective separation of the larger particles from smaller particles.
Some typical particle classifiers are disclosed in U.S. Pat. Nos. 2,485,255; 2,868,462; and 3,098,036. However, most such classifiers have not been able to provide a desired degree of control of the particle sizes. In many classifiers, radially oriented vanes are usually initially set and are seldom changed due to the low effectiveness of such vanes in controlling the coal particle size. However, these particle size control problems have been substantially overcome by use of the present invention, which uses two sets of flow control vanes in a series flow arrangement.